The goal of preventing neuronal degeneration in a clinical setting requires that each major class of genes controlling neuronal survival be characterized. Neurotrophic factors function via tyrosine kinases. Little is known about the 'other half' of this fundamental mechanism: tyrosine phosphatases. This gene family i likely to have a fundamental parallel role in regulating neuronal survival and development. The LAR tyrosine phosphatase receptor is a prototype for this gene family. The discovery by the investigators that several LAR splice variants are developmentally regulated and preferentially expressed by neurons and associated cells suggests that certain LAR isoforms modulate neural survival and development. Demonstrating neurotrophic effects of LAR will first require that they determine which lAR isoforms are most relevant to neuronal development, sprouting and regeneration. They can then express these isoforms and test their ability to modulate neuronal survival, neurite outgrowth and neuronal response to NGF. Hypothesis: The LAR tyrosine phosphatase receptor represents a new category of genes regulating neuronal survival and development. Neurons and associated cells express specific LAR isoforms which regulate development, sprouting and regeneration. Recombinant proteins derived from this subset of LAR isoforms can be used to control neuronal survival. Specific Aims: 1) Use quantitative RT-PCR to establish which LAR isoforms are regulated during neuronal regeneration and sprouting. These isoforms are the ones most likely to influence neuronal survival or neurite outgrowth. 2) Determine by in situ hybridization/immunohistochemistry which neurons express LAR isoforms regulated during regeneration and sprouting. These studies will suggest which neurons are influenced by particular LAR isoforms. 3) Isolate a full-length cDNA clone corresponding to a developmental-regulated LAR splice variant which is regulated by denervation and which predicts a secreted LAR isoform. Since LAR may function via homophilic interactions (i.e., act as its own ligand), a secreted LAR isoform will be a powerful tool for examining LAR effects on neuronal survival and neurite outgrowth. 4) Use LAR clones corresponding to LAR transcripts preferentially expressed in the nervous system and associated with develop, regeneration and sprouting to make plasmid constructs for expressing these LAR protein isoforms. Demonstrate in well-established in vitro models that these LAR protein isoforms regulate neuronal survival and/or neurite outgrowth.